Node 20 LTS reaches end-of-life April 2026. Updated all references:
- Dockerfile base image to node:22
- All GitHub Actions workflows to Node 22
- Bumped actions/checkout and actions/setup-node to v4 where outdated
- Added .nvmrc file for local development
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>